Various processes for producing extruded synthetic resin foams hitherto have been known.
Extruded foams are generally produced from a synthetic resin by a so-called extrusion process, i.e. by adding a cell controlling agent to the synthetic resin, kneading the mixture in a molten state with heating, admixing a volatile-type foaming agent (physical blowing agent) with the kneaded mixture, and extruding the resulting mixture into a low-pressure zone to produce a foam by the evaporation of the foaming agent.
Among the foams produced by the above process, the extruded foam of polystyrene is widely used as a heat-insulating material and the like because the foam has a high expansion ratio and a uniform cell structure, and exhibits excellent heat-insulating properties and suitable mechanical strength.
However, since the polystyrene foam prepared by the extrusion process is composed primarily of polystyrene of great rigidity as the base resin, the foam is low in flexibility and still remains to be improved in flexibility.
It is therefore considered useful to adjust the amount of the cell controlling agent and the composition of the foaming agent to be used to give a uniform and subtle cell structure to extruded foams of a synthetic resin such as polystyrene and thereby impart improved flexibility to the foam.
Nevertheless, an attempt to simply decrease the cell size increases the density of the foam under the influence of the surface tension of the cell membrane, so that difficulties are encountered in preparing a synthetic resin foam having a uniform and fine cell structure of low density.
Further if the conventional extrusion process is used for preparing a foam of considerably increased thickness with a reduced cell size, it is known that the cells become more anisotropic in the direction of thickness of the foam to result in impaired heat-insulating properties and lower flexural strength.
In fact, we attempted to prepare a polystyrene resin foam of uniform and fine cell structure by the extrusion process with controlled amounts of foaming agent and cell controlling agent, whereas the decrease of the cell size to not greater than 0.4 mm produced more anisotropic cells, failing to impart the contemplated heat-insulating properties and flexural strength to the foam obtained. When a further increased amount of the cell controlling agent was used to decrease the cell size to 0.2 to 0.3 mm, it became difficult to foam the resin material in the direction of thickness of the foam to be obtained, consequently affording a foam of greatly reduced thickness and entailing the problem that it was difficult to obtain a foam of satisfactory thickness.
Besides the foregoing process, it is also known to produce a foam having different cell sizes by an extrusion process wherein water is forced directly into molten resin within an extruder to foam the resin by the evaporation of water and cool the molten resin with the latent heat of evaporation of water (Japanese Unexamined Patent Publication No. 176226/1983). Since this process employs no means for holding the water micro-scopically dispersed in the molten resin, the water forced in cannot be thoroughly dispersed in the resin, permitting formation of coarse cells and giving a poor appearance to the product. Although this process appears to afford a foam of a cellular structure having large and small cells in combination, cells of different sizes are conjointly present only in a small quantity locally of the cell structure. Thus, the process in no way provides a foam, wherein cells of different sizes are conjointly present as dispersed approximately uniformly, which is consequently outstanding in heat-insulating properties and bending deflection, as contemplated by the present invention.
Another process has been disclosed for producing a foam of a cell structure with different cell sizes using a water-containing vegetable substance 75 to 500 .mu.m in particle size, especially pulverized corncobs, to ensure uniform dispersion of water (Japanese Unexamined Patent Publication No. 236839/1986). However, the disclosed process is not satisfactory for the following reasons. When the starting mixture is fed to an extruder, the water-containing vegetable organic substance releases much water. The use of the natural substance inevitably gives a product of unstable quality. Moreover, the difficulty encountered in accurately controlling the water content produces irreguralities in the cell structure of the foam extruded. Furthermore, the vegetable organic substance is carbonized during heating and melting within the extruder to color the extruded foam brown and cause the foam to give off an odor.
Under the foregoing situation, we developed foams of cell structure wherein cell membrane form large cells and small cells which are conjointly present as dispersed like islands in the sea (Japanese Patent Application Nos. 170928/1989 and 201018/1989). These foams retain the heat-insulating properties and suitable flexural strength required for foams, exhibit high flexibility and are therefore free from all the foregoing problems.
We have conducted intensive research in order to further improve the thermal properties of the foams previously developed and unexpectedly found that when water is forced into a mixture of the synthetic resin and a water-absorbent high molecular compound along with an evaporable foaming agent after the mixture has been kneaded in a molten state with heating, small cells can be made still smaller to impart improved heat-insulating properties to the resulting foam.
Our research further carried out has revealed that in the case where a phenolic antioxidant is mixed with the synthetic resin along with the water-absorbing high polymer compound, and the volatile-type foaming agent is forced into the mixture with addition of water during the mixing, or while or after the mixture is kneaded in a molten state with heating, it becomes easy to form many small cells with higher stability than in case of the above foam, consequently affording greater freedom in setting the production conditions, making possible the production of a more satisfactory foam with higher stability and giving improved stable heat-insulating properties to the foam. The present invention has been accomplished based on these findings.